Colorimeters are often used to determine the color of an existing object, such as a paint sample, so that the color can be reproduced. In order to reproduce color so that it appears as a duplication to the human eye, a light source used during the process of measuring color must correlate to the spectral sensitivity of the human eye. As is well known, a human eye has three different kinds of color receptors (cones) that are sensitive to various spectral bands or regions that roughly correspond to red, green, and blue light. The receptors are relatively "broadband" devices, sensitive to a wide range of wavelengths within each color band region. For example, as shown by the solid line curves 12, 14, and 16 in FIG. 1, blue receptors are typically sensitive to light having wavelengths ranging from about 400 nm to 500 nm (curve 12); green receptors are sensitive to light having wavelengths ranging from about 480 nm to 630 nm (curve 14); and red receptors are sensitive to light having wavelengths ranging from about 500 nm to 660 nm (curve 16). While the specific sensitivities of the color receptors vary from person-to-person, the average response for each receptor has been quantified and is known as the "CIE standard observer." The three intensity-versus-wavelength curves are referred to as the CIE standard X, Y, and Z tristimulus functions where the X function curve 16 relates to red light, the Y function curve 14 relates to green light, and the Z function curve 12 relates to blue light.
Accurately reproducing colors in a tristimulus system is accomplished by ensuring that the light sources used to illuminate the desired object have spectral bands or ranges that match as closely as possible to the spectral response ranges of the three receptors (i.e., red, green, and blue) in the human eye. Since the receptors of the human eye are sensitive to a relatively broad spectral range of light colors, the light sources used in colorimeter devices must have similar broad spectral ranges if accurate color reproduction is to be achieved.
Many techniques, including light separation and light filtering, have been implemented in colorimeters to generate light that tracks the tristimulus values of the human eye. In the light separation technique, polychromatic light (usually white light) is separated into multiple color component beams, each of which is then focused onto a dedicated photo sensor. For example, a single light source is split using an optical prism into red, green, and blue color component portions which are then simultaneously projected onto three separate linear photo sensors. The output from each photo sensor represents the tristimulus value for the corresponding primary color. Although this technique works well for its intended purpose, the separation of the initial light source requires optical devices that add cost to a calorimeter.
With regard to the light filtering technique, polychromatic light (usually white light) is projected onto a sample and then the light that reflects from the sample is divided into three substantially identical beams. Each of the three reflected beams is then passed through one of a blue, green, or red filter/photo sensor combination. Within the filter/photo sensor combination through which a reflected beam is passed, the beam is filtered by a color-specific filter in order to obtain the respective one of the three tristimulus values as an output of the photo sensor. Although this technique works well for its intended purpose, the filtering of light requires additional optical devices to be integrated into a colorimeter for carrying out the light filtering. In addition, filters used to mimic tristimulus values may not closely match the actual X, Y, and Z tristimulus functions.
As an alternative to utilizing polychromatic light sources and manipulating light to coincide with the CIE standard X, Y, and Z tristimulus functions, colored LEDs have been used to generate red, green, and blue light. Traditionally, LEDs have efficiently generated red light, with green and blue light being generated by doping LEDs to shift the emitted wavelength. Doping of LEDs has generated poor results when trying to match the broadband intensity-versus-wavelength curves of the CIE standard X, Y, and Z tristimulus functions. The dashed line curves 22, 24, and 26 of FIG. 1 show how typical doped LEDs match up with the established CIE standard X, Y, and Z tristimulus functions. As can be seen, the closest match between corresponding curves is between the blue light LED curve 22 and the Z tristimulus function curve 12, while the green and red light LED curves 24 and 26 and the Y and X tristimulus function curves 14 and 16, respectively, are less closely matched. Unfortunately, it is difficult to fabricate light sources and/or filters having broadband spectral ranges that can closely approximate those of human receptors, much less provide an identical match.
In another approach, instead of trying to dope LEDs to match the CIE standard X, Y, and Z tristimulus functions, LEDs generating light over a broad range of wavelengths are mixed to try to match the desired intensity curves. Mixing of light from LEDs requires additional systems that are not necessary if the LEDs are able to generate light with spectral distributions that initially match the CIE standard X, Y, and Z tristimulus functions.
In view of the shortcomings involved with manipulating light to match the CIE standard X, Y, and Z tristimulus functions, what is needed is a light source that more closely matches the CIE standard X, Y, and Z tristimulus functions, with the light source being of the type that can be integrated into a calorimeter so that color can be measured in a manner that better correlates to the color sensitivity of the human eye.